In a photovoltaic module, the photovoltaic cells may be connected in series by means of an electrically conducting element, such as a copper strip, which connects the front face of a cell to the rear face of the adjacent cell.
In such a module, the front face of the different cells is situated on the same side, forming the front face of the module, and the rear face is situated on the opposite side, forming the rear face of the module.
FIG. 1 illustrates this interconnection mode, referred to as “standard mode”, in a partial sectional view of a module comprising cells C1, C2, C3.
The front face of the cells is designated by reference sign AV, the rear face is designated by reference sign AR.
To connect the front face of the cell C1 to the rear face of the cell C2, the copper strip 1 is not planar but crosses the module from the front face to the rear face.
Once the different cells are connected by means of the strips 1,1′, etc. they are encapsulated in an encapsulating material 2 and laminated between two glass panels 3, 3′, or between a glass panel on the front face and a polymer panel on the rear face (said polymer being able to be transparent or not according to whether the module is monofacial or bifacial).
The crossing of the strip from the front face to the rear face requires a deformation of the strip, which is capable of creating mechanical stresses in said strip, favouring chemical corrosion or instead mechanical fatigue of said strip and thus causing electrical (rupture of interconnections) or mechanical (fissuring) failures of said module.
This interconnection mode applies not just to monofacial cells (that is to say of which only one of the main faces is photoactive) but also to bifacial cells (of which the two main faces are photoactive). Such bifacial cells may be obtained by only metallizing locally the rear face of a conventional cell, for example in the form of a grid or any other form.
In the case of a module comprising bifacial cells, another possible interconnection mode is a so-called “monolithic” interconnection, shown schematically in FIG. 2.
Reference signs identical to those of FIG. 1 designate the same elements as those already described with reference to this figure.
In this interconnection mode, the cells are arranged according to the + and − polarity of the connections connecting the front face AV of a cell and the rear face AR of the adjacent cell.
This makes it possible to use a flat copper strip 1,1′ to connect respectively the front face of the cell C1 to the rear face of the adjacent cell C2, on the front face of the module, and the front face of the cell C2 to the rear face of the adjacent cell C3, on the rear face of the module.
Such an interconnection mode is described for example in the patent DD 135 014.
The monolithic interconnection has the advantage of making it possible to connect simultaneously all the cells of the module, unlike the standard interconnection mode which includes a plurality of steps, the cells being connected successively to each other. The method for producing the module is moreover simplified by the fact that no prior deformation of the copper strips is necessary.
FIGS. 3A and 3B illustrate in a schematic manner the sequence for interconnecting cells respectively in the case of a standard interconnection and a monolithic interconnection.
In the case of a standard interconnection (FIG. 3A), on account of the conformation of the copper strip, a cell can only be assembled when the copper strip has been connected to the adjacent cell. The assembly thus requires a succession of steps E1, E2, E3, etc. until all of the cells have been connected.
In the case of a monolithic interconnection (FIG. 3B), the assembly is carried out in a single step, two series of copper strips (one on the side of the front face of the module, the other on the side of the rear face of the module) being simultaneously connected to the corresponding faces of the cells.
Another advantage of monolithic interconnection is that it minimizes stresses in the copper strips and thus limits the risks of failure linked to said strips.
Yet another advantage of monolithic interconnection is that the spacing of the cells may be minimized. In fact, in standard interconnection, a certain spacing of the cells is necessary to enable the crossing of the copper strip from the front face to the rear face. Such a spacing is largely reduced, thus increasing the area efficiency (W/m2) of the module, in the case of a monolithic interconnection.
It is in addition known that bifacial photovoltaic cells have a different conversion efficiency between the front face and the rear face. This difference is due on the one hand to the physical properties of the material forming the cell and on the other hand to the presence of a denser metallization on the side of the rear face than on the side of the front face. This difference may also result from the choice of optimizing the efficiency of one of the two faces to the detriment of the other.
FIG. 4 illustrates as an example the conversion efficiency R (Internal Quantum Efficiency) of the front face (curve a) and of the rear face (curve b) of a bifacial cell as a function of the wavelength A.
Generally, in bifacial cells currently present on the market, the ratio between the conversion efficiency between the front face and the rear face is of the order of 70% to 95%.
In a photovoltaic module comprising a plurality of cells, the electric current generated depends on the cell that produces the least current.
Consequently, in a monolithic arrangement as described above, the electric current produced by the module is only 70 to 95% of the current that would have been produced by the module if said module had been assembled according to the standard interconnection mode.